feat(map): fetch all schools for map view, add reference pin, cap radius at 5mi

- Remove 10-mile radius option; cap backend radius max at 5 miles
- Raise backend page_size max to 500 so map can fetch all schools in one call
- HomeView: when map view is active, fetch all schools within radius
  (page_size=500) instead of showing only the paginated first page;
  falls back to initial SSR schools while loading
- SchoolMap/LeafletMapInner: accept referencePoint prop and render a
  distinctive coral circle pin at the search postcode location

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.py

@@ -293,9 +293,9 @@ async def get_schools(
     school_type: Optional[str] = Query(None, description="Filter by school type", max_length=100),
     phase: Optional[str] = Query(None, description="Filter by phase: primary, secondary, all-through", max_length=50),
     postcode: Optional[str] = Query(None, description="Search near postcode", max_length=10),
-    radius: float = Query(5.0, ge=0.1, le=50, description="Search radius in miles"),
+    radius: float = Query(5.0, ge=0.1, le=5, description="Search radius in miles"),
     page: int = Query(1, ge=1, le=1000, description="Page number"),
-    page_size: int = Query(25, ge=1, le=100, description="Results per page"),
+    page_size: int = Query(25, ge=1, le=500, description="Results per page"),
     gender: Optional[str] = Query(None, description="Filter by gender (Mixed/Boys/Girls)", max_length=50),
     admissions_policy: Optional[str] = Query(None, description="Filter by admissions policy", max_length=100),
     has_sixth_form: Optional[str] = Query(None, description="Filter by sixth form presence: yes/no", max_length=3),